FORMATION AND EVOLUTION OF TIN SURFACE DEFECTS DURING CYCLIC MECHANICAL LOADING
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Stress relaxation in tin films can result in microstructural changes visible on the surface, referred to as “surface defects,” and can include whisker and hillock formation, cracking, nucleation of new grains, and grain growth. Sn whiskers are of particular concern for microelectronics reliability in which Sn whiskers growing from component surface and cause catastrophic short-circuiting. While prior research has identified the conditions and mechanisms for surface defect evolution during aging and thermal cycling, the response of tin films due to mechanical stress, especially high frequency vibration, is not fully understood. In practical terms, high frequency vibration is an important source of mechanical stress generation in microelectronics for automotive and aerospace applications. This research, based on high frequency vibration of cantilevers, adds to the existing mechanisms for stress relaxation process in metal thin films, not just for tin films, as well as proposed new mechanisms for such processes.
In the first study, the piezoelectric drive of small atomic force microscopy (AFM) cantilevers vibrated at resonance are used for high frequency cyclic bending experiments. Intermetallic (IMC) formation as well as initial film morphology and thickness (corresponding to surface grain size) all influence the response of tin films for cyclic bending. A laser doppler vibrometer (LDV) system was used to identify the real-time strain along the cantilever during cycling, suggesting that the small strains are responsible for the limited nucleation and growth for defects though the defect density increases with the number of cycles and strain distribution along the cantilever.
In the second study, the effect of larger strains on defect evolution was determined using vibration of larger cantilevers at resonance as a function of number of cycles, frequency, temperature, and whether the vibration was continuous or interrupted for SEM characterization of defect type and density. In addition to typical micro-sized whiskers and hillocks, intragranular breakup (IGB) with intrusions and extrusions and nanowhiskers (NWs) with diameters < 1 𝜇m were observed. Both increasing number of cycles and strain amplitude/rate promote defect formation for a fixed frequency, with the defect density being strongly frequency dependent.Vibration at low temperature and interrupting measurements for SEM characterization affected the relative densities. The density of larger surface defects is strongly influenced by interruptions while NW density is almost unaffected.
Both low resonant frequency and low T (223 K) promote IGB formation during cyclic bending due to large maximum strain amplitude and slower diffusion/creep at low T, respectively. Though the overall defect density for low T is smaller than that at room temperature (RT), the response of films is similar to that at RT, indicating the same mechanisms. The defect density decrease at low T is mainly determined by NW formation, and there is a transition from micro-sized surface defects to IGBs for cyclic bending at low T.
This research demonstrated that cyclic bending of cantilevers can be used to quantify the stress relaxation of tin films in an important stress regime for microelectronics and to develop defect mitigation strategies to improve the reliability of interconnects in electronic applications.